The present invention generally relates to power system protection, and more specifically, to an apparatus and method for determining a faulted phase of a three-phase ungrounded power system.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include protective devices and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
Such protective devices and procedures act to isolate some power system element(s) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s).
Power system protection may be grouped into six types including: (1) generators and generator-transformer elements (2) transformers, (3) buses, (4) lines (transmission, sub-transmission and distribution), (5) utilization equipment (motors, static loads), and (6) capacitor or reactor banks. As a result, a variety of protective devices are required. Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosers.
Although the fundaments of power system protection are similar, each of the six types of power system protection use protective devices that are based on the characteristics of the power system elements in that category. More specifically, different protective relays utilizing a variety of protective schemes (e.g., differential current comparisons, magnitude comparisons, frequency sensing), are required to protect the various power system elements.
For example, an overcurrent relay is designed to provide overcurrent protection against faults occurring on a distribution line of an ungrounded power system. Using power system current information, if the overcurrent relay senses a current that exceeds a threshold, it sends a trip signal to a power circuit breaker which then opens to isolate the faulty distribution line (faulty line) from the remainder of the power system.
Because power system currents can easily exceed 10,000 amperes (amps) and power system voltages can reach several hundred thousand volts, and because a protective device, such as the overcurrent relay described above, is designed to measure currents no greater than 100 amps and measure voltages no greater than a few hundred volts, the protective device is coupled to the power system element(s) via a number of current and/or voltage transformers (CT and VT). The CT and VT may be arranged in one of a number of arrangements (e.g., grounded wye, ungrounded delta). During operation, the current transformers and voltage transformers proportionally step-down the power system voltage and current (while retaining the same phase relation) flowing into the protection zone, to a magnitude that can be readily monitored and measured by the protective device.
As is known, the three-phase current flowing into the protected line is commonly referred to as a primary current, and the current flowing from the current transformers to the protective device is commonly referred to as a secondary current. Likewise the voltage present on the protected line is commonly referred to as primary voltage, and the voltage delivered to the protective device is commonly referred to as secondary voltage. When received by the protective device, the resulting lower secondary currents and voltages can be filtered, sampled, etc., to determine corresponding phasors representative of the primary voltages and primary current flowing into the protected line. The phasors are then used in the various protective relay logic schemes, such as, for example, in the overcurrent scheme of the overcurrent relay to calculate the magnitude of the currents.
In some instances the protective device must not only detect the presence of a power system fault, but must also determine which of the three power system phases is faulted. When the power system is grounded, determining the faulted phase often is as easy as determining which phase conductor carries the largest current. When the power system is ungrounded, determining the faulted phase is often as easy as determining which phase conductor carries the smallest voltage with respect to ground (the phase-to-ground voltage). In actual practice however, three-phase ungrounded power systems typically employ open delta voltage transformer banks to provide stepped-down phase-to-phase voltages to the protective device, and broken delta voltage transformer banks to provide zero-sequence voltage to the protective device. These typical transformer arrangements do not allow the protective device to directly measure the phase-to-ground voltage of the protected line.